Aluminum Coordination and Lewis Acidity in Aluminas and Steamed Zeolites

Abstract

27Al MAS NMR spectra obtained upon cross-polarization (CP) show that the distorted coordination shells of AlIV and AlV are surface species in small slabs of dehydroxylated alumina. These AlV-rich slabs are present in steamed zeolite as non-framework aluminum or in transition aluminas obtained from ground boehmite. P(CH3)3 is strongly bound to the Lewis site associated with AlV and this Lewis site is strong, as evidenced by the shift of the 31P resonance. This shift goes from −51 ppm, in alumina without AlV, to +5 ppm for the AlV −P(CH3)3 adduct. Furthermore, a thorough kinetic study of the model reaction: 1-butene ↝cis and trans 2-butene shows that the Lewis sites associated with AlV enhances the cis ↽trans conversion in transition aluminas. These direct correlations support the earlier indirect evidences that AlV yields strong Lewis sites.