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

Abstract

The topological symmetry of the as-synthesized ZSM-5 is orthorhombic Pnma, with 12 crystallographically independent tetrahedral sites in the unit cell. Highly crystallized ZSM-5 zeolite can be synthesized using organic species as structure directing agents (SDA), including tetra-n-propylammonium (TPA) and Na cations. This work investigates the thermal stability and high temperature structural evolution by in situ synchrotron (X-Ray) powder diffraction of TPA-ZSM-5 with different Si/Al ratios (15, 25, 50, and 100) and their structural modifications induced by template calcination. Rietveld refinements of the room temperature data revealed that in all samples tetrapropylammonium ions are located at the intersection of the sinusoidal and the straight channels. The refined TPA+ content increases with the increasing of the Si/Al ratio of the zeolite, reaching the theoretical value of 4 when the Si/Al ratio = 84, in very good agreement with both chemical and thermogravimetric analyses. The bond distances evidenced the lack of interactions between the TPA+ and the framework oxygen atoms. The high temperature refinements confirmed the high thermal stability of all four ZSM-5 samples, which remain crystalline until the end of the thermal treatment (800 °C). Burning of the template does not cause any relevant variations in the framework geometry and channel shape, confirming the high flexibility of the MFI framework.In addition, thermal analysis proved insights about the effect of aluminum content on the interaction between SDA and zeolite structure.