A DRIFT spectroscopic study of the N 2 adsorption and acidity of H-faujasites

Abstract

Brønsted acidity of faujasite (FAU) type zeolites, such as, H-Y, dealuminated H-Y, and ultra-stabilized H-Y (USY) was studied by DRIFT spectroscopy using N 2 adsorption at 298 K and at pressures ranging from 1 to 9 bar for probing of the acidic sorption sites. Both the α- and β-cage hydroxyl-groups were found to establish H-bonds with N 2. Dealumination by (NH 4) 2[SiF 6] or ultra-stabilization increased the accessibility of the β-cage protons. The Δ ν OH red shift, induced by the adsorption, was used as a measure of the intrinsic acid strength of the OH-groups. Samples, free of extra-framework aluminum (EFAl) species, had about the same Brønsted acid strength regardless of their Al content. In the EFAl-containing USY sample a fraction of the hydroxyl groups showed enhanced Brønsted acidity.