COMPUTER-SIMULATION OF ZEOLITE STRUCTURE AND REACTIVITY USING EMBEDDED-CLUSTER METHODS

The use of bare cluster models to understand the nature of zeolite-sub strate interactions may be improved to take account of the environment of the Bronsted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves gene rating a specialised correction potential by fitting a non-periodic ar ray of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic arra y of atomic charges to the potential of the infinite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM clus ter is embedded. Such embedded cluster calculations, employing a T3 cl uster, with electron correlation at the density functional theory leve l, are described, to model the interaction of water at a Bronsted acid site. Structures of the water-zeolite complex, and associated vibrati onal frequencies and H-1 NMR shifts are calculated and compared with c alculations of bare clusters of varying size and with experimental dat a. We then describe a mixed quantum mechanical-molecular mechanical (Q M-MM) model derived by combining charges from the second model with a standard aluminosilicate force held. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbo n cracking reaction; the methyl-shift reaction of a propenium ion coor dinated to the acid site.