Determination of structure and acidity scales in zeolite systems by ab initio and pseudopotential calculations

Abstract

SCF ab initio calculations at the 6-31G level have been used to investigate the structure of several aggregates simulating some of the proton donor sites within faujasite-type zeolites. The Si(OH)/sub 4/, H/sub 3/SiOHAlH/sub 3/, and (OH)/sub 3/SiOHAl(OH)/sub 3/ clusters have been successively examined. Deprotonation energies and charge distribution are determined at a higher level by using a 6-31G basis set augmented with polarization and diffuse functions. The results are compared with values obtained by using pseudopotential methods. The small differences between the two sets of results demonstrate that comparable accuracy should be expected from both procedures. Finally, deprotonation energies of (OH)/sub 3/T/sub 1/OHT/sub 2/(OH)/sub 3/ aggregates (T/sub 1/, T/sub 2/ = AlSi, BSi, GaSi; AlGe, BGe, GaGe) are calculated by using pseudopotential methods and compared with the results given by the semiempirical MNDO method. In some cases ab initio SCF calculations were also performed. The results confirm that the inclusion of boron atom lowers the acidity as already demonstrated by experimental investigation. The effects due to the inclusion of Ga are discussed and compared to available experimental data.