Cluster and periodic ab initio study of the ethane-ethene hydride transfer reaction catalyzed by acid chabazite.: Is the cluster model able to describe accurately the host-guest interactions?

The ethane-ethene hydride transfer reaction on acid chabazite is used to test the performance of cluster calculations with respect to periodic ones for the study of bimolecular hydrocarbon reactions on acid zeolites. The following computational levels are considered: Hartree-Fock, MP2, a posteriori density functional theory (DFT) correlation using the PW91 functional, and several DFT hybrid B3PW91 procedures with different percentages of Hartree-Fock exchange. In all cases, a polarized basis set of double-zeta quality is used. Three periodic models with different numbers of acid sites and adsorbed substrates are explored to account for the effect of Si : Al ratio and coverage. Cluster and periodic calculations at different computational levels have been performed using the local geometries obtained from cluster constrained optimizations at the B3PW91 level. Our results indicate that the electrostatic field generated by the crystalline framework has only a relative influence on the stability of charged intermediates, that depends on the coverage and composition of the model, and also on the computational method used. It is noted in particular that the additional stability provided by the crystalline Madelung potential on the charged intermediate increases with the percentage of Hartree-Fock exchange in the DFT hybrid B3 scheme.