AB-INITIO MOLECULAR-ORBITAL CLUSTER STUDIES OF THE ZEOLITE ZSM-5 .1. PROTON AFFINITIES

In this study we carried out ab initio molecular orbital calculations using the STO-3G, 3-21G, and 6-31G* basis sets on aluminosilicate clusters representing ZSM-5 containing up to 46 tetrahedrally coordinated tetravalent atoms and including (SiO)n rings. The proton affinity and OH stretching frequency of the central bridging oxygen site (O(24) site) are examined as a function of cluster size and shape. The proton affinity is found to depend significantly (5-15 kcal/mol) on the shape of the cluster. Calculation of the electrostatic potential in the region of the central bridging oxygen site for the clusters indicates that this dependence is largely due to electrostatic effects. The effects of geometry relaxation are studied with a constant-volume relaxation method in which a part of the cluster is optimized with the remaining being fixed in its crystal structure. Results indicate that relaxation increases the proton affinity by about 10-15 kcal/mol over the value from the rigid experimental structure. The final calculated proton affinity for the O(24) site in ZSM-5, including correction for zero-point effects, O:Si ratio, and higher level theory, is consistent with experiment. The OH stretching vibrational frequency is in good agreement with the experimental value.