HZSM-5 Zeolites with Different SiO₂/Al₂O₃Ratios. Characterization and NH₃Desorption Kinetics

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This article reports the influence of the SiO 2 /Al 2 O 3 ratio (30, 80, and 280) on HZSM-5 zeolite properties. XRD patterns show a decrease in the XRD peak intensity at low 2θ angles for low SiO 2 /Al 2 O 3 ratio zeolites. N 2 adsorption and desorption isotherms display hysteresis with both micropores and mesopores likely contributing to the particles network. The NLDFT cylindrical model confirms the characteristic 5.5 Å micropores in the zeolite pore framework. Furthermore, specific surface area, pore volume, and pore size distributions demonstrate that changes in the SiO 2 /Al 2 O 3 ratio do not influence HZSM-5 structural properties. NH 3 -TPD shows both weak and strong acid sites. Pyridine-FTIR also confirms that, in HZSM-5, the weak acidity encompasses Lewis and hydrogen-bonded sites, while the strong acidity mainly involves the Brönsted acid sites. It is proven that the ratio of weak to strong acid sites as measured by NH 3 -TPD and Pyridine-FTIR displays similar changes to the SiO 2 /Al 2 O 3 ratio. Furthermore, NH 3 desorption kinetics and numerical regression allow the establishment of energies of activation for both strong and weak acid sites. It is also found that the kinetic desorption rate constants vary in agreement with the changes in weak and strong acid sites as observed with NH 3 -TPD and Pyridine-FTIR.