The structural and chemical influences on catalytic properties of zeolite clusters

Abstract

The structural and chemical effects on the catalytic properties of zeolite clusters were investigated within the frameworks of ab initio (3-2IG, DZ, DZP and TZ2P) and semiempirical (MNDO, AM1 and PM3) methods. Full optimizations under the constraints of C1, Cs, C2, C2h, D3dand D6hsymmetry point groups were carried out for the different types of SiO(H)Al units in the secondary building units of the zeolite cluster models SixAlyO30H12 + y(x/y = 12/0, 11/1, 10/2, 9/3, 8/4 and 6/6). Changes in the environment of the silicon and aluminium framework atoms with a given Si Al ratio generate new different acid sites. A consequence of these acidity changes is also partly due to the changes in bridging hydroxyl group (SiOHAl) angles. All semiempirical and ab initio SCF models of Si10Al2O30H14demonstrate that Dempsey's rule may be incorrect. On the basis of the charges on the hydroxyl proton, the OH bond distance, and the ionicity of OH bond, a strong correlation was established between the Brønsted acidity and the amount as well as the location of aluminium atoms. The results of the ab initio, MNDO and AM1 calculations, but not the PM3 calculations, indicate that the Brønsted acidity may increase with a decrease in aluminium content, in accordance with recent experimental data. © 1995.