Active site structure of Cu/ZSM-5: Computational study

Abstract

The coordination structures of Cu ionic species in ZSM-5 zeolite were investigated by molecular dynamics (MD) simulations and molecular orbital (MO) calculations. Al sites were first sought so as to reproduce the square-planar coordination of Cu(II), which is well characterized by the ESR spectra. When two Al's occupy T8 sites in the six-membered ring, the calculated coordination of Cu(II) agreed well with the experimental data. For this Al siting, Cu(II)-O-Cu(II) and two Cu(I)'s were respectively simulated, and the coordination of the former species was found to be consistent with the EXAFS results. Furthermore, from the MO calculation Cu-O-Cu was anticipated to be very reactive and thus might be the active site. The function of zeolite would therefore be the stabilization of this species. On the other hand, Cu dimer was found unstable, and the reaction mechanism based on it was supposed unlikely.