Ab-initio molecular orbital calculations on the siting of aluminium in the Theta-1 framework: Some general guidelines governing the site preferences of aluminium in zeolites

Abstract

Ab-initio molecular orbital calculations are utilized to obtain the preferred tetrahedral (T) sites for substitution of Si by AI in the Theta 1-framework. Using a method developed by Derouane and Fripiat ( Zeolites 1985, 5, 165), it is shown that the T1 and T4 sites of Theta 1 are the favored sites for A1 substitution. A survey of preferred Al substitution sites for Theta 1, ZSM-5, and ferrierite indicates that Al preferentially substitutes into sites having the highest average TO bond lengths and the lowest average TOT bond angles. It is suggested, however, that the use of the crystallographic TO bond lengths may lead to errors in the substitution trend, and, hence, it is preferable to discuss favored Al substitution sites in terms of TOT angles alone. Analysis of preferred Al substitution sites as the sole criteria of average TOT angle around a particular T site is shown to give rise to a slightly different substitution pattern than that obtained via the method proposed by Derouane and Fripiat.