NH3-TPD and FTIR spectroscopy of pyridine adsorption studies for characterization of Ag- and Cu-exchanged X zeolites

Pure crystalline NaX zeolites with Si/Al = 1.2 is prepared by the hydrothermal method. The modification is achieved by following the conventional ion-exchange technique to obtain Ag- and Cu-exchanged forms of NaX zeolite with different metal loadings ranging from low to high exchange. Compositional and structural investigations of all samples are performed by atomic absorption spectrometry, elemental analysis, powder XRD, and nitrogen adsorption porosimetry. The acidity of all exchanged zeolites is investigated using both the ammonia temperature-programmed desorption (NH₃-TPD) carried out by microbalance, and the pyridine adsorption performed via Fourier transform infrared spectroscopy (FTIR). XRD analysis shows the global conservation of the zeolitic structure after ionic exchange. The crystallinity loss is more important for zeolites exchanged at high degree of copper. Porosity measurements indicate a decrease of the porous volumes and the specific surface areas when the Ag or Cu exchange degree is raised. Mesopores particularly appear at high exchange degree of copper. NH₃-TPD indicates that the increase of Ag and Cu-exchange degrees leads to an increase of the global acidity, which is more pronounced for Cu-exchanged zeolites. The strength of acid sites for Ag-exchanged zeolites is weaker than for Cu-exchanged zeolites. The FTIR spectroscopy analysis by pyridine adsorption confirms the formation of extra framework aluminum (EFAL) species, inducing an increase in the concentration and the strength of the Lewis acidity. The formation of EFAL is more important for Cu-exchanged zeolites.