Characterization of Unmodified and Zinc-modified ZSM-5 Zeolites with Temperature-programmed Desorption of Ammonia and Isopropylamine

Abstract

While the temperature-programmed desorption of ammonia (NH3-TPD) is widely used to analyze the combined Lewis and Brønsted acidity of heterogeneous catalysts, the temperature-programmed desorption of isopropylamine (IPAm-TPD) can be used for the selective analysis of Brønsted acidity. This work compared NH3-TPD and IPAm-TPD as analysis methods for the acidity of zeolitic samples, including H-ZSM-5-23, H-ZSM-5-50, H-ZSM-5-280, a Zn-modified H-ZSM-5-50 sample, and a γ-Al2O3 reference sample. For the unmodified H-ZSM-5-23, H-ZSM-5-50, and H-ZSM-5-280, the total acidity determined with NH3-TPD remained higher and the Brønsted acidity determined with IPAm-TPD lower than the theoretical acidity estimated with the Al content of the materials. When the NH3-TPD saturation temperature was varied for H-ZSM-5-50 to examine the trends observed in the analyses, the temperature change affected primarily the low temperature peak of the TPD traces. The Zn-modified Zn/H-ZSM-5-50 sample yielded a multi-peak IPAm-TPD trace, though only a single peak trace was expected. Additionally, the value of Brønsted acidity showed no change from the unmodified zeolite.