The Strength of Brønsted Acid Sites in Microporous Aluminosilicates

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.