Acid sites in dehydroxylated Y zeolites: an X-ray photoelectron and infrared spectroscopic study using pyridine as a probe molecule

Abstract

N 1s XPS spectra of pyridine chemisorbed on HY zeolites have been obtained from samples calcined at temperatures ranging from 300 to 700 °C. HY zeolite samples calcined at 300 and 400 °C showed two N 1s XPS peaks at 402.4 and 400.4 eV, which are assigned to Brönsted and Lewis sites, respectively. However, for samples calcined at 500 °C and above, three N 1s components with binding energy values 402.4 ± 0.2, 400.6 ± 0.3 and 399.4 ± 0.2 eV were observed. The present results suggest that the lowest binding energy peak arises from the pyridine chemisorption on relatively weak Lewis sites that are formed during dehydroxylation. The N 1s binding energy of this Lewis component depends upon the zeolite calcination temperature. An infrared study of the hydroxyl region revealed bands at 3544, 3644 and 3740 cm^–1 for 300 and 400 °C calcined samples. New weak bands at 3695, 3665 and 3595 cm^–1 appeared for samples calcined above 500 °C. Comparison of the results suggested that the species responsible for the IR band at 3665 cm^–1 upon adsorption of pyridine showed the N 1s XPS peak at 399.4 ± 0.2 eV. The shift in the binding energy towards a lower value for the 700 °C calcined sample suggests that the species responsible for this band are thermally unstable and are transformed into a relatively low acid strength Al₂O₃ phase.