ABINITIO QUANTUM CHEMICAL CALCULATIONS OF ALUMINUM SUBSTITUTION IN ZEOLITE ZSM-5

We have performed ab initio quantum mechanical calculations in monomeric clusters modeling the 12 different T sites of zeolite ZSM-5. By comparing the results of calculations that use minimum basis sets with those that employ valence double-zeta bases, we conclude that minimum basis sets are unreliable for predicting relative replacement energies for the substitution of silicon by aluminum atoms at the T sites of the zeolite. From these calculations, we also conclude that small differences in the bond lengths and angles can significantly alter the order of the sites with respect to the replacement energies. From calculations using valence double-zeta basis sets on T(OH)4 monomers, we conclude that, in the absence of protons or other ions, the most favorable sites for Al substitution in zeolite ZSM-5 are the T6, T12, and T9 sites, whereas the least favorable site is T3. However, the least favorable and most favorable sites only differ by 3.3 kcal/mol. We also present a simple empirical model that is capable of reproducing the results of the ab initio calculations. This model gives the replacement energy in terms of the bond lengths and bond angles about each site.