CHARACTERIZATION OF ACID SITES IN BETA AND ZSM-20 ZEOLITES
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Beta (Si/Al = 5-20) and ZSM-20 zeolites with low Si/Al ratios have been synthesized, using tetraethylammonium hydroxide as an organic template, and characterized by XRD, SEM, TGA, and XPS. The X-ray diffraction study indicated that an analcime-like phase crystallizes along with zeolite Beta when the starting mole ratio Si/Al 5. X-ray photoelectron spectroscopy (XPS) showed that a single-component Gaussian-shaped Al(2p) peak is associated with the framework Al species present in pure (no other crystalline and/or amorphous phase) Beta, ZSM-20, and HY zeolites. The presence of amorphous Al species produced a shoulder peak to the main Al(2p) peak at low-binding-energy values, whereas the presence of Al species associated with the other zeolitic structure gave a shoulder peak at high-binding-energy values. The acidic properties of these zeolites were studied by temperature-programmed desorption (TPD) of pyridine, FTIR spectra in the hydroxyl as well as pyridine region, and X-ray photoelectron spectra of chemisorbed pyridine. TPD of pyridine revealed that the chemisorbed pyridine was desorbed at somewhat higher temperature from Beta zeolites as compared to ZSM-20 and/or HY zeolites. The N(1s) XPS spectra of pyridine chemisorbed on these materials indicated that the Beta, ZSM-20, and STY (steam-treated Y) zeolites contain three N(1s) component peaks, whereas the HY zeolite spectrum yielded two N(1s) component peaks. The N(1s) binding energy data suggested that acid sites present in Beta, ZSM-20, and STY zeolites are stronger than those in HY zeolites. An IR study of zeolite Beta showed only one acidic hydroxyl band at 3602 cm-1, whereas ZSM-20 showed classical high-frequency (3644-cm-1) and low-frequency (3542-cm-1) bands similar to those observed in HY zeolite. The IR spectra obtained at a pyridine desorption temperature of 450C revealed that most of the pyridine was desorbed from Brnsted acid sites of HY zeolite but not from the Brnsted acid sites of Beta and ZSM-20 zeolites. From the comparison of these results with those obtained with HY zeolite, the following order for their acidic strength of the framework hydroxyls is derived: Beta > STY > ZSM-20 HY. The n-hexane cracking activity, a representative Brnsted acid catalyzed reaction, was found to be consistent with the derived acidity order. 1992 American Chemical Society.
