PREFERRED STABILITY OF AL-O-SI-O-AL LINKAGES IN HIGH-SILICA ZEOLITE CATALYSTS - THEORETICAL PREDICTIONS CONTRARY TO DEMPSEYS RULE

The question of which factors, beyond Lowenstein's rule, determine the Al distribution within the frameworks of high-silica zeolite catalysts has not been settled. Lattice energy minimizations based on classical potentials and large scale quantum chemical ab initio calculations show that two Al atoms preferentially occupy two next-nearest-neighbor tetrahedral sites in four-membered rings of H-faujasites and of other protonated frameworks containing double-six-membered ring building units. This finding is surprising, since it was previously believed that the Al atoms assume the largest possible distance for a given Si/Al ratio (Dempsey's rule).