AB-INITIO SCF-MO STUDY OF THE TOPOLOGY OF THE CHARGE-DISTRIBUTION OF ACID SITES OF ZEOLITES

The structural, electronic, vibrational, acid topologic properties of a series of acid sites of zeolites were studied at different levels of ab initio molecular orbital theory. The zeolite acid sites were model ed by using the following molecular clusters: silanol H3SiOH (B-0) and the clusters H3SiO(H)AlH3 (B-1), (OH)(3)SiO(H)Al(OH)(3) (B-1_OH), and H3SiO(H)Al(OH)(2)SiH3(B-2). The calculation of geometries and propert ies of these clusters were performed at the Hartree-Fock level, and, a dditionally, second-order Moller-Plesset (MP2) and density functional BLYP calculations were carried out for silanol and B1 clusters. Geomet ries were fully optimized by following Cs symmetry restrictions. The s tandard STO/6-31 + G(D, P) basis set, which includes polarization and diffuse functions, was used for all the calculations. The topologic pr operties of the zeolite acid clusters, based on the theory of atoms in molecules, were analyzed in terms of the total density and the Laplac ian density properties, both evaluated at the position of the bond cri tical points. The calculations showed that the frequency of the OH vib rational modes of the zeolite acid sites, often used as an infrared in dex for characterizing the acidity of zeolites, is Linearly related to the total density of the charge at the critical points of the OH bond s, with a correlation coefficient of r(2) = 0.97. These results indica te that the total density of the electronic charge at the critical poi nt of the OH bond can be used as a tool for interpreting the structura l and electronic features of the zeolite hydroxyl groups. A relationsh ip between the Mulliken population of the H atom of the OH bond and th e OH frequency gives a correlation coefficient of 0.67. On the other h and, the values of the Laplacian density calculated at the critical po ints of the bonds of the acid sites indicate that the zeolite structur e is dominated by a network of Si-O and Al-O ionic interactions, while the O-H bonds are characterized as covalent bonds, with different ext ents of charge concentration. (C) 1998 John Wiley & Sons, Inc.