Toward an Understanding of Zeolite Y as a Cracking Catalyst with the Use of Periodic Charge Equilibration

Periodic charge equilibration (PQEq) is used to study a few of the roles that the zeolite Y lattice might play in cracking catalysis. Comparison of the partial charge distributions of HY and Na 1 HY suggests that atoms within 10 Å of the sodium atom accept electrons from the sodium atom. The charge distributions of n -octane, benzene, and water adsorbed inside the supercage of zeolite Y and siliceous zeolite Y show that the lattice provides little perturbation to these molecules. In contrast, a local dipole in the zeolite Y lattice is found to induce a change in the partial charge distribution of a cracking hydride transfer transition state model (C 4 H 9 + plus n -octane). The polarization in this transition state model is attributed to the increased size of the transition-state model in comparision to the size of n -octane. This polarization likely preferentially stabilizes species such as the transition state and is perhaps an explanation for the second-order dependence of hydrocarbon cracking rate on Al ion concentration in zeolite Y.