The Strength of Brønsted Acid Sites in Microporous Aluminosilicates

Abstract

Ensemble-averaged deprotonation energies (DPE) derived from periodic density functional theory models are insensitive to the location of isolated Al atoms and associated protons and similar among microporous aluminosilicates (i.e., zeolites) with different crystalline frameworks (MFI, BEA, FER, MOR, CHA, FAU). These DPE values are 1201 ± 11 kJ mol–1 after correcting for systematic artifacts of periodic DFT methods, which vary with framework density, and averaging over the four distinct proton locations at each Al atom. These energies rigorously reflect the strength of the acid sites in these important catalytic solids. Thus, the stability of the conjugate anions and the acid strength of these materials merely reflect the presence of Al atoms within the silicate framework, and not their specific siting or local confining environment. DPE values did not show any systematic trends with the vibrational frequency or length of O–H bonds, with Si–O–Al bond angles, or with NH3 adsorption enthalpies, properties that are frequently but inaccurately used as experimental indicators of acid strength. Such properties may reflect or bring forth confinement effects that do not influence acid strength, but which can stabilize the relevant ion-pair transition states and adsorbed intermediates through dispersion forces. These findings confirm that the different shape and size of the confining voids near Al atoms and their associated protons, instead of any differences in their acid strength, give rise to the remarkable diversity of acid forms of zeolites in the practice of catalysis.