Computer simulation of zeolite structure and reactivity using embedded cluster methods

Abstract

The use of bare cluster models to understand the nature of zeolite–substrate interactions may be improved to take account of the environment of the Brønsted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves generating a specialised correction potential by fitting a non-periodic array of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic array 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 cluster is embedded. Such embedded cluster calculations, employing a T3 cluster, with electron correlation at the density functional theory level, are described, to model the interaction of water at a Brønsted acid site. Structures of the water–zeolite complex, and associated vibrational frequencies and ¹H NMR shifts are calculated and compared with calculations of bare clusters of varying size and with experimental data. We then describe a mixed quantum mechanical–molecular mechanical (QM–MM) model derived by combining charges from the second model with a standard aluminosilicate force field. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbon cracking reaction; the methyl-shift reaction of a propenium ion coordinated to the acid site.