Hybrid quantum chemical and density functional theory (ONIOM) study of the acid sites in zeolite ZSM-5

The three-layer ONIOM(B3LYP/6-311+G**:HF/3-21G*:MNDO) method was used to model the structural, energetic, and spectroscopic (IR and 1H NMR) properties of the acid sites (types 1 and 2) of zeolite ZSM-5. The calculation method used reproduced the properties of studied acid sites well. In the case of the second type of Bronsted acid site, the hydrogen of the bridged hydroxyl group forms a hydrogen bond with the next nearest oxygen to the aluminum atom. The analysis of experimental IR and 1H NMR spectra in combination with current calculations suggests that the experimentally observable mean acidity of zeolite ZSM-5 depends on temperature because the occupancies of protons at various oxygen sites are kinetically controlled and thermodynamic equilibrium is reached at high temperatures.