Non-empirical quantum-chemical calculations on ZSM-5 zeolites. I. Brønsted acid sites

This paper describes the study of the -OH group in ZSM-5 zeolite. The importance of zeolite ZSM-5 and the advantages in studying the OH groups by computer simulation techniques are highlighted. The methodology followed in the present study, which is a combination of classical and quantum-chemical calculations, is briefly described. The basis set SV3-21G has been chosen following trial calculations performed with different standard sets of basis functions. Results of geometrical optimizations carried out to determine the position of hydrogen in the ZSM-5 lattice are reported. The size of the point-charge (PC) cluster is varied and its effect on the calculated properties of the hydroxyl group is discussed. Calculations on a larger quantum-chemical (QC) cluster containing the hydroxyl group in both normalized and experimental structures reveal that geometry is an important factor for electronic properties. The procedure for generation of a suitable PC cluster to represent the rest of the lattice is described. Results of a calculation for a cluster model of 82 ions (PC cluster) in which a Si2O7 (QC cluster) entity is explicitly treated by ab initio procedures, are discussed. The reported parameters include the O-H vibrational frequency, the proton binding energy and the aluminium substitution energy and we discuss their implications for the catalytic properties.