Statistical analysis of Al distributions and metal ion pairing probabilities in zeolites

The distributions of Al tetrahedral sites are studied as a function of Si : Al ratio (from 11 to 95) in three zeolites: ZSM-5, ferrierite, and mordenite. Al configurations obeying Loewenstein's rule are generated by a Monte Carlo procedure and analyzed in terms of accessibility to specific channels. The distribution of pairs of Al sites accessible to the same channel segregates into distinct nearest-neighbor and ring-bridging types. Such pairs provide favorable binding environments for extraframework MOxM2+ cations in the metal-ion-exchanged forms of the zeolites. In all three zeolites, the probability of a given Al site having at least one suitable partner for pairing within 8.5 Angstrom is close to unity for typical Si : Al ratios. The probability at shorter distances is more sensitive to the zeolite structure, reflecting the larger number of highly puckered five- and six-membered rings in ZSM-5. This sensitivity, and its implications for CuOxCu2+ formation in the three Cu-exchanged forms, may explain why Cu-ZSM-5 is the most active of the three for catalytically decomposing NO.