Infrared spectroscopic study of the acid properties of dealuminated BEA zeolites

Abstract

A series of HBEA samples were prepared by dealumination through three different methods (steaming, treatments with hydrochloric acid and ammonium hexafluorosilicate (HFS) solutions) of a parent sample with total and framework Si:Al ratios of 12.5 and 33, respectively, constituted of 20 nm crystallites. The samples were characterized by XRD, nitrogen adsorption and pyridine adsorption–desorption followed by IR spectroscopy. Whereas the three dealumination treatments have practically no effect on crystallinity and nitrogen adsorption properties, they cause large changes in the ranges of the O–H bond stretching modes (3300–3800 cm −1) and of the most intense IR absorption of pyridine (1400–1700 cm −1). Treatments with HCl or HFS cause the most significant changes: disappearance of the 3662 and 3782 cm −1 bands corresponding to extraframework Al species and to tricoordinated Al species partially connected to the framework, respectively, appearance of additional bands at 1603 and 1446 cm −1 ascribed to a new type of pyridine species coordinated to Lewis sites (PyL 2) and for desorption temperatures above 350 °C, of a band at 1462 cm −1 generally ascribed to iminium ions, apparently at the expense of the PyL 2 band. In agreement with this formation of iminium ions, desorption at high temperatures causes the complete disappearance of pyridinium ions without complete restoration of the acidic bridging hydroxyl band. PyL 2 species are proposed to be pyridine molecules coordinated to Lewis sites and interacting through hydrogen bond with protonic sites.