Calculation of the properties of acid sites of the zeolite ZSM-5 using ONIOM method

Abstract

The performance of the ONIOM method was tested in reproducing the properties of the acid sites of the zeolite ZSM-5. The acid sites in three crystallographic locations (Al3–O19(H)–Si6, Al6–O18(H)–Si9 and Al7–O17(H)–Si4) have been modelled by three combinations of two-layer ONIOM method with varied size (two and eight T atoms) and calculation method (ab initio and DFT) used for the chemically important (active) part. The best correspondence with experimental data was achieved in case of Al6–O18(H)–Si9 site calculated with ONIOM(B3LYP/6-311+G ∗∗:MNDO) method using eight T atoms in model system for the following properties: stretching vibrational frequencies, 1H NMR chemical shifts of the bridged hydroxyl groups, vibrational frequency shifts and changes in 1H NMR chemical shifts of the hydroxyl groups upon adsorption of CO.