Acidity of the Br�nsted acid sites of zeolites

Abstract

The acidity of Brönsted acid sites of the (H3SiO)nOq(X3−n−qSi)(OH)AlpY3−m−q(OSiH2)r(OSiH3)m zeolitic clusters, with X=(H,OH), Y=(H,OH), (n,m)=(0,1,3), r=(0,1,8), and (p,q)=(0,1), is evaluated in terms of the intrinsic acidity þ of the OH groups. þ is defined as the ratio between the OH distance and the frequency of the OH vibration mode and has been calculated at ab initio Hartree–Fock (HF) and second-order Møller–Plesset (MP2) levels and the standard 6-31+G(d,p) basis set. The acidity þ is rationalized in terms of the topology of the charge density ρ(r) calculated at the position of the critical point of the OH bond ρ(rc) of the acid sites. It is shown that there exists a linear relationship between þ and the charge concentration ρ(rc), with a correlation coefficient of r=−0.98 for HF results, and r=−0.96 for MP2 ones. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 143–150, 2001