PNNL

Abstract

We have performed density functional theory calculations to describe the interaction of an active site of the HZSM-05 zeolite with a benzene molecule. We used a ring of ten SiO4 tetrahedral (10T) units to represent the structure of the HZSM-5 zeolite. The calculations were of the all-electron type, the exchange-correlation contributions were taken into account by means of the BLYP density functional, and orbital basis sets of DNP quality were employed for all atoms. Starting from the silicalite 10T ring, with silicon atoms at each site, we found the most energetically favored site for the substitution of an aluminum atom by a silicon atom to produce an HZSM-5 ring model, with a Si/Al ratio of nine. In order to simulate the adsorbed state of benzene onto the zeolite, the geometry of the aromatic molecule was fully optimized in its interaction with the zeiolite model, while keeping the ring frozen. The electronic structure of the benzene-HZSM-5 complex was then analyzed and discussed. Our results account for a significant interaction between the acid proton from the HZSM-5 cavity with benzene, showed by changes of the X-bond cloud of benzene, which would lead to an active carbocationic moiety.