An in-situ synchrotron X-ray powder diffraction study of the thermal stability of templated ZSM-5 zeolites

Organic species such as tetra-n-propylammonium (TPA) and Na cations have been found to be effective in directing the structure of ZSM-5 zeolite. The optimization of synthesis conditions is crucial to producing highly crystalline ZSM-5 zeolite and, consequently, to controlling the properties of the resulting material for specific applications. ZSM-5 actual symmetry (orthorhombic Pnma, with 12 crystallographically independent tetrahedral sites in the unit cell) depends on the synthesis and post-synthesis treatment, SiO2/Al2O3 ratio, structural defects, temperature, and amount of sorbate organic molecules. Through in situ synchrotron powder diffraction of four TPA-ZSM-5 samples with varying Si/Al ratios (15, 25, 50, and 100), this work examines the thermal stability and high temperature structure development. By performing Rietveld refinements on the room temperature data, it was discovered that the TPA+ ions are situated at the junction of the sinusoidal and straight channels in every ZSM-5 sample. The refined TPA+ content increases with the increasing Si/Al ratio of the zeolite. All four ZSM-5 samples have good thermal stability, as demonstrated by the high temperature refinements; they stay crystalline until the thermal treatment reaches its conclusion (800 °C). The great degree of flexibility of the MFI framework is confirmed by the fact that burning the template does not result in any discernible changes to the channel form or framework geometry. Furthermore, thermal studies provided valuable insights into how aluminum presence affects the way TPA and zeolite structures interact.