Role of Defects in Pore Formation in MFI Zeolites

Silicalite crystals, crystallized with fluoride ions in the synthesis mixture, are stable in alkaline solution for at least 1 week of base treatment. Nuclear magnetic reasonance (NMR) and infrared (IR) spectroscopy evidence that silicalite is essentially free of defect sites. MFI crystals with decreasing aluminum content in the order ZSM-5 (Si/Al = 14) > ZSM-5 (Si/Al = 50) > silicalite were base leached. Based on X-ray diffraction (XRD) patterns, only silicalite and ZSM-5 (Si/Al = 14) retain the MFI morphology for as long as 1 week in alkaline solution (0.1 M NaOH at 80 °C), while that of ZSM-5 with Si/Al = 50 is completely destroyed. The modulating effect of aluminum sites is excluded in silicalite, and its lack of defects explains the stability. Scanning electron microscopy (SEM) images show that leaching takes place in the center of the crystals preferentially in the [010] direction. The preferential pore formation indicates that structural properties have an effect on creation of porosity upon alkaline treatment.